Overview

This how-to is intended to give a decent description on how to add dropdown form elements to the backend admin page for a component. I worked out the steps primarily on my own by dissecting/replicating the Banners component. In particular, the 'banners' section of com_banners - not! to be confused with 'banner').

In general, the process of adding these fields breaks down into the following steps:

Creating the class that will generate the form element

Adapting the view to display the form element

Adapting the model to retrieve data from DB & to set submission values in the Joomla 'state'

Frontend component note

I have followed and tested this approach and it's almost the exact same for the frontend component.Might want to change and redirect this article's title though (with maybe some changes?).
(E-builds)

Extend JFormFieldList (models / fields / fieldname.php)

Frequently your filter fields will be very basic; probably simply a dropdown list of one of the columns being displayed on the current page. Regardless, you will need to create your own field element. This really isn't a big deal - this file will probably be less than 70 lines, including comments. The code below will generate the dropdown element to filter by companies. For more information and complex examples on creating this class see Creating_a_custom_form_field_type.

Obviously, there is only one section here that needs to be customized:

$query->select('id As value, name As text');$query->from('#__my_companies AS a');$query->order('a.name');$query->where('state = 1');

Change the database name and adjust the sql as necessary. This will produce the text and values for the select element options. I highly recommend not changing the value and text names. Although I did not test this theory, it seems likely that ancestor classes are expecting these names as keys in the resulting array.

Modify View (views / zzz / view.html.php)

In you view class, you have to set the state, like this:

$this->items=$this->get('Items');$this->pagination=$this->get('Pagination');$this->state=$this->get('State');//Following variables used more than once$this->sortColumn=$this->state->get('list.ordering');$this->sortDirection=$this->state->get('list.direction');$this->searchterms=$this->state->get('filter.search');

Extra: highlighting search terms

Here is some extra to visually mark the found search terms in the results page.

Template

Add this somewhere at top somehwere:

$searchterms=$this->state->get('filter.search');//Highlight search terms with js (if we did a search => more performant and otherwise crash)if(strlen($searchterms)>1) JHtml::_('behavior.highlighter',explode(' ',$searchterms));

And further in your template, enclose the specific fields with the default highlighter finder.